1. Field of the Invention
The present invention relates to an image signal converting apparatus suitable for so-called up-conversion where an image signal with a standard resolution is converted into an image signal with a high resolution.
2. Description of the Prior Art
For example, in the case of TV signals, in addition to standard resolution (or standard definition SD) TV signals, high definition (or high definition HD) TV signals have been experimentally used for broadcasting. Further, TV receivers that can receive HD signals have been practically used. In other words, the HD TV system is likely going to coexist with the SD TV system.
When the SD TV system and the HD TV system coexist, a signal converting apparatus for allowing an HD TV monitor to reproduce SD TV video signals will be required. Although various standards for the SD signals and the HD signals have been proposed, now assume an NTSC system for SD signals (number of scanning lines=525, number of fields=60, aspect ratio=4:3) and an HDTV system for HD signals (number of scanning lines=1125, number of fields=60, aspect ratio=16:9). The resolution of the HD signals is four times higher than the resolution of the SD signals. Thus, the converting apparatus should perform an up-converting process that increases the number of pixels of the input SD signal by four times so as to obtain the resolution of an HD signal.
Conventionally, the signal converting apparatus uses interpolating filters. FIG. 1 shows a block diagram of a conventional signal up-converting apparatus. An SD signal is supplied from an input terminal 1. The SD signal is sent to a horizontal interpolating filter 2 that doubles the number of pixels of the SD signal in the horizontal direction. The output of the horizontal interpolating filter 2 is sent to a vertical interpolating filter 3 that doubles the number of pixels of the output signal of the horizontal interpolating filter 2 in the vertical direction. The output of the vertical interpolating filter 3 is an HD signal. The HD signal is sent to an output terminal 4. In the conventional signal converting apparatus, the image is up-converted with such filters. For example, 0 data is inserted into an interpolating point so that the sampling frequency of the SD signal accords with the sampling frequency of the HD signal. An interpolated value corresponding to the interpolating point is formed by each filter.
FIG. 2 shows a construction of each of the interpolating filters. An SD signal is supplied to an input terminal 5. The SD signal is sent to multipliers that multiply the SD signal by filter coefficients .alpha..sub.n, .alpha..sub.n-1, . . . .alpha..sub.0. Each of outputs of the multipliers is sent to a register with a unit delay amount T. The output of each of the multipliers and the output of each of the registers T are added. An interpolated output is sent to an output terminal 6. In the horizontal interpolating filter 2, the unit delay amount T accords with a sample period. In the vertical interpolating filter 3, the unit delay amount T accords with a line period.
In the conventional image signal converting apparatus, since the SD signal is up-converted into the HD signal with the filters, the output HD signal is generated by processing the input SD signal. Thus, the resolution of the output HD signal is not higher than the resolution of the input SD signal.
An object of the present invention is to provide an image signal converting apparatus that can compensate the resolution of the output signal.
Another object of the present invention is to provide an image signal converting apparatus that categorizes the input signal as classes corresponding to local features of the input image signal so as to improve conversion accuracy.